In pressurized medium systems, actuators which perform rotary or linear motions, such as motors and cylinders, are utilized in a known manner. In controlling the actuators, valves are utilized for controlling the pressurized medium for example to control the direction of movement of the actuator or to keep it stationary. Different functions of the actuators are implemented by suitable couplings of different valves.
Several different couplings for controlling an actuator are implemented particularly in directional valves comprising a spool which is axially movable in a bore in the valve body and which, in its different positions, couples the different connections and ports of the valves in a desired way. The different channels of the valve are connected to the connections and ports. Typically, the different channels include a pressure port P, one or more work ports, such as a work port A and a work port B, and a tank port T. The actuator is coupled to one or more work ports by means of pipes or hoses.
The spool comprises control edges which throttle the volume flow. The pressure difference prevailing across the control edges of the spool, together with the cross-sectional area of the flow (the aperture of the valve), determine the quantity of pressurized medium flowing through the valve, that is, the volume flow from one channel to another. The cross-sectional area of the flow depends on the position of the spool in relation to the valve body and the channels. By the design of the control edges of the spool, for example various grooves and bevels, it is possible to affect the way in which the cross-sectional area of the flow changes from one position of the spool to another.
The pressure in the work port of the valve may vary as the load of the actuator changes. As a result, the pressure difference prevailing across the control edge changes, whereby the volume flow changes as well, if the cross-sectional area of the flow remains constant. It is also possible that the feeding pressure in the pressure port may vary for any reason. In view of the control and steerability of the actuator, however, it is often necessary that the volume flow remains constant so that e.g. the speed of the actuator does not change. Thus, so-called pressure compensation is possible.
Conventionally, the pressure compensation of the directional valve is implemented with a separate pressure valve used as a compensator and connected to the control circuit to which the directional valve belongs. The compensator valve is placed, for example, in a line coupled to the pressure port or the tank port of the directional valve. The compensator valve tends to maintain a given pressure difference across the control edge of the directional valve, wherein the variation in the volume flow can be compensated for. In an ideal case, the flow can be maintained constant.
Particularly in larger valves, the compensator valve is a complex, bulky, heavy and costly component.